Neuronal cell death takes place both during normal development and in various pathological conditions such as epilepsy, Alzheimer's disease, and brain ischemia. We have found that tissue plasminogen activator (tPA), a serine protease that converts plasminogen to plasmin, mediates neuronal cell death in the hippocampus. In addition, tPA mediates activation of microglia, the "immune" cells of the brain. tPA has long been known to be expressed under basal conditions by neurons. After excitotoxic injury, however, we have found that the primary source of hippocampal tPA becomes newly activated microglia. We propose to: 1. Identify tPA/plasmin substrate(s) that mediate physiological function. tPA is expressed in the mammalian brain under physiological conditions and is secreted during neuronal remodeling. Preliminary data suggest that tPA/plasmin are involved in mossy fiber sprouting after stimulation of neuronal activity. Chondroitin sulfate proteoglycans represent promising candidates and will be analyzed further. In addition, chemokines will be evaluated as potential targets for tPA/plasmin action. 2. Dissect the neuronal cell death pathway mediated by tPA. Our preliminary evidence suggests that tPA mediates excitotoxic death in the hippocampus via apoptosis. We will confirm this observation and define the pathway of signaling, including potential role(s) played by microglia. 3. Determine whether tPA is required for neurodegeneration in a related setting. A mutant mouse that displays spontaneous neuronal degeneration will be outcrossed to homozygous tPA-/- mice. Their progeny will then be used to determine whether the absence of tPA confers protection to the neurons at risk in the mutant genotype. 4. Investigate the involvement of tPA in human neurodegeneration. The autoimmune disease multiple sclerosis (MS) is characterized in part by a marked increase in activated microglia, high levels of tPA activity that mirror disease progression, and neurodegeneration. We propose to use an MS animal model to evaluate the potential role of tPA in MS, in order to determine whether it is likely to be a requisite part of the neurodegenerative pathway, or is likely to be useful solely as a marker for disease progression. The aim will include validating the model by correlating changes in levels of tPA activity with the disease progression, and manipulating tPA activity levels to determine if the disease progression can be altered.